The use of magnetically-actuated indicator dials with float-type liquid level gauges is known. A receiving magnet connected to a dial indicator is magnetically coupled (typically through a gauge head) to a second magnet that moves in response to a change in liquid level in the tank. Such gauges are used with liquefied petroleum gases such as propane, butane and the like referred to herein as LPG. One such gauge is disclosed in U.S. Pat. No. 6,041,650 issued Mar. 28, 2002 to Swindler et al., the disclosure of which is incorporated herein by reference for all purposes.
A major advantage of magnetically actuated dials is that such dials do not require a mechanical coupling extending through the wall of a tank. However, the magnetic field of the receiving magnet must freely interact with the magnetic field of the second magnet. Thus, the material separating the receiving magnet from the second magnet must be substantially non-magnetic. For purposes of this application, the term “non-magnetic” refers to materials having a relative magnetic permeability (compared to vacuum), μr, similar to that of non-ferrous metal materials such as aluminum, copper, zinc, brass, and austenitic stainless steels, and substantially lower than that of ferrous metal materials. Consequently, prior art gauge heads have been fabricated from non-magnetic materials, typically brass. These gauge heads are typically fabricated as a single-piece unit adapted to be threadedly engaged with a corresponding fitting in the tank. These gauge heads are configured to receive a magnetically actuated dial on the exterior of the tank with a second magnet mechanically coupled to a float assembly inside the tank.
For a variety of reasons, e.g., material cost, a single-piece gauge head fabricated entirely from a non-magnetic material is not always desirable. Thus, there exists a need for a gauge head that can be fabricated in part from a ferrous metal material that still provides a low magnetic permeability “window” through which the receiving magnet of a magnetic dial and a second magnet can be magnetically coupled.